A. Technical Field
The present invention relates to: a microcapsule for an electrophoretic display device in which an electrophoretic fine particle and a solvent are encapsulated in a shell; a process for manufacturing the same; and a sheet for an electrophoretic display device using the microcapsule.
B. Background Art
An electrophoretic display device is a non-emission-type display device using electrophoretic phenomenon of a pigment particle in a dispersion liquid in which an electrophoresing pigment fine particle is dispersed in a coloring solvent. More particularly, the electrophoretic display device has hitherto been known as a device having a structure in which the dispersion liquid is encapsulated into a space provided between counter electrode substrates (films), at least one of which is transparent, in which a voltage is applied to a prescribed position between both electrode substrates to electrophores an electrophoretic fine particle, and display is performed utilizing a difference in an optical density generated between other position, and has many excellent properties such as wide viewing angle property, long time memorizing property without power (continuous supply) and low power consumption.
In recent years, instead of the prior electrophoretic display device in which the aforementioned dispersion liquid is encapsulated as it is into a space between counter electrode substrates (for example, see patent document 1 below), an electrophoretic display device having a structure in which microcapsules in which the aforementioned dispersion liquid is encapsulated in a shell (capsule shell; also referred to as wall film, the same hereinafter) as a wall material are spread between counter electrode substrates (for example, see patent documents 2 and 3 below) are positively developed and researched, and various performances and functions such as long stability of display, responsiveness, contrast and rewritable times of display are greatly improved as compared with the aforementioned prior electrophoretic display device.
As microcapsulating technique which can be applied when a microcapsule for an electrophoretic display device using the aforementioned dispersion liquid as a core substance is prepared, there are a so-called interface segmentation method such as a coacervation method (phase separation method) (for example, see patent document 4 below), a melting degradation cooling method and a powder bed method, and a so-called interface reaction method such as an interface polymerization method, an in-situ method, an in-liquid curing film (covering) method (orifice method) and an interface reaction method (inorganic chemistry reaction method). Inter alia, a coacervation method is generally suitable since the method has advantages that a strength and a thickness of a shell are easily controlled, and a shell of plural layers can be formed. For example, a microcapsule having a shell obtained from gelatin and gum arabic as essential raw materials by the coacervation method is well known as a microcapsule for an electrophoretic display device (for example, see patent document 2 below).
When an electrophoretic display devise using a microcapsule is tried to be applied to a variety of display devices in the various utility fields, further improvement in performance in various performances such as contrast greatly influencing on clearness of an image is earnestly desired.                [Patent Document 1] JP-B-015115/1975 (Kokoku)        [Patent Document 2] Japanese Patent No. 2551783        [Patent Document 3] JP-A-086116/1989 (Kokai)        [Patent Document 4] U.S. Pat. No. 2,800,457        
However, the prior electrophoretic display device using a microcapsule has the problem that, when allowed to stand under the high temperature and high humidity condition, in particular, when allowed to stand under the high temperature and high humidity condition for a long time (e.g. under 60° C., 90% RH for 24 hours), remarkable reduction in contrast thereafter is recognized.